Flexible micro-/macro-electronics that are light-weight, robust, and capable of being folded or rolled up for easy carriage, storage, and attaching to uneven surfaces have been an important pursuit of research in the past decade. Some of the potential applications, such as flexible displays and radio-frequency identification tags, require modest electrical performance of the active TFTs, while low cost and the capability of large-area fabrication are the primary concerns. See, for example, Gernier et al., Science 265, 1682 91994); Drury et al., Appl. Phys. Lett. 73, 108 91998); and Voss, Nature 407, 422 (2000). For these applications, flexible electronics using organic semiconductors, amorphous silicon (α-Si), and poly-crystal silicon (poly-Si) have shown tremendous promise. See, for example, Baude et al., Appl. Phys. Lett. 82, 3964 (2003); Gelinck et al., Nature Mater. 3, 106 (2004); Chen et al., Nature 423, 136 (2003); and Chuang et al., Proceedings of SPIE 5801, 234 (2005).
On the other end of the scale, however, lie applications that require reasonable cost but high performance radio frequency (RF) circuitry and can operate in the several hundred MHz to multiple GHz and even tens of GHz regimes. One example is the large military active antenna for surveillance systems, unmanned aerial vehicles and space-based radars. See, for example, Reuss et al, Proceedings of the IEEE 93, 1239 (2005). Flexible electronics are ideal for these applications since low weight, small stow volume, and the ability to conform to complex shapes are crucial. The active circuitry for these antennas are desirably operated in the ultra high frequency (UHF) (−500 MHz) and higher RF frequency range (several GHz). Such a demand on higher operation frequencies has made it impractical to use organic semiconductors, α-Si and poly-Si in TFT applications.
The mobility of electrons and holes in organic materials is rather low. As a result, high-performance (high speed and low power consumption) flexible electronics cannot be realized with these materials, despite the fact that these materials are easy to use on flexible substrates (e.g., can be printed or spun-on at low-temperatures). Similarly, due to poor microstructure, the mobility of α-Si is also very low (<10 cm2/V-s) and prevents it from being used in high-performance applications. The mobility of poly-Si is much higher than that of α-Si; however, it requires higher processing temperatures than that of α-Si. The required higher-temperature processing of poly-Si has excluded it from being used on a number of desirable low-temperature substrates, like plastic (e.g., PET and PEN). Furthermore, the mobility of poly-Si is still not high enough for this type of material to be used in many high-performance, very-large-area applications.
Currently, the large-area antennas mentioned above are built by hardwiring the high-performance; stand-alone units which are fabricated using highly developed integrated circuit (IC) technology. It is not until recently that high quality, single-crystal semiconductors have been monolithically integrated onto flexible polymer substrates. See, for example, Tilke et al, Appl. Phys. Lett. 77, 558 (2000); Menard et al., Appl. Phys. Lett. 84, 5398 (2004); Menard et al., Appl. Phys. Lett. 86, 093507 (2005); and Sun et al, Appl. Phys. Lett. 87, 083501 (2005). Electron mobility as high as 270 cm2/V-sec has been demonstrated on TFTs using a single-crystal Si channel with improved ohmic contacts for the source and drain. See, for example, Zhu et al, Appl. Phys. Lett. 86, 133507 (2005). However, to date, no flexible Si-based TFTs have been demonstrated capable of operating at RF frequencies (e.g., >1 GHz).
Recently, advances on releasing and transferring single-crystal Si template layers from silicon-on-insulator (SOI) substrates have opened new possibilities for employing single-crystal silicon in flexible electronics. However, many significant challenges need to be overcome before any practical application employing these new materials can be realized. First, the commercial SOI substrates, which are designed for advanced complementary metal-oxide semiconductor (CMOS) field-effect transistor applications (e.g., microprocessors), are greater than 4 to 6 times more expensive than regular Si substrates. Consequently, the essential requirement of low cost for flexible electronics using these materials cannot not be satisfied. The low-cost requirement will also not be attained by using other materials that are not mass-produced, like Si(111) substrates, because the custom manufacturing of these substrates also implies high cost. Besides the availability issue of Si(111), the lower carrier mobility and higher interface states will lower the device speed, and the current complicated material processing procedures of Si(111) over Si(100) will present significant disadvantages of employing the Si(111) substrates for low-cost flexible electronics. Furthermore, the material process for Si(111) substrates for TFT fabrication is not compatible with the commercial Si CMOS process. Thus, a need exists for a method that is compatible with the Si CMOS process and capable of deriving a single-crystal Si layer from low-cost, commercially produced silicon substrates, such as Si(100) and Si(110).